Aqueous solutions that contain one or more dissolved metals in ionic form may be subjected to solvent extraction for the recovery of one or more desired metals. The desired metal ions are usually extracted from aqueous solution into an organic solvent containing an extractant and are recovered from the loaded solvent by stripping with a suitable aqueous strip solution. The other metals, present as ions in the aqueous solution as impurities, must often be removed from the process as they may cause difficulties in the stripping of the desired metal, and often increase in concentration in the circulating solvent to an extent that affects the efficiency of the extraction process.
Methods that are used alone and in combinations for removing desired and impurity cations present in solvent extraction processes include the conventional stripping or selective stripping with acidic or basic solutions and the more recently developed hydrogen reductive stripping, hydrolytic stripping and electrolytic stripping.
Stripping is often accomplished with acids or bases under ambient or elevated conditions. Hydrogen reductive stripping is carried out at temperatures between 150.degree. and 350.degree. C. under elevated pressure and usually in the presence of seed metal particles to produce a metal powder. In hydrolytic stripping, loaded solvent is subjected to elevated temperatures (100.degree.-250.degree. C.) in the presence of water whereby metal oxides or hydroxides are formed. Hydrogen reductive stripping and hydrolytic stripping have been reviewed by Monhemius, A. J., Mintek 50, pp. 599-609. Electrolytic stripping has been applied to a loaded solvent by subjecting the solvent to electrolysis with electrodes placed in the loaded solvent (Wan, R. Y., et al., J. of Metals, Dec. 1986, pp. 35-40).
Ferric iron may also be stripped from various loaded organic solvents into the aqueous phase with an acid alone or combined with the introduction of sulfur dioxide or hydrogen sulfide to reduce ferric to ferrous. The stripping may be carried out at ambient or elevated temperatures and pressures. It is noted that iron is usually present in solvent extraction processes as ferric and that, in many cases, ferrous is the stable form in the aqueous phase.
The above prior art methods have a number of disadvantages. In conventional stripping, high concentrations of the strip solution are often required. Where lower concentrations are used, the processes are complicated by, for example, the use of combinations of extractants. Hydrogen precipitation and gaseous and/or hydrolytic stripping, especially under elevated pressure and at higher temperatures, are expensive and complex.
In aqueous hydrometallurgical processes use is often made of galvanic reactions between metals that cause reduction of a metal cation and precipitation, i.e. cementation, onto an added solid metal. This has not been applied to solvent extraction processes. Any methods disclosed for the reduction of metal cations to reduce and cement metal cations onto an added solid metal have been applied before carrying out the solvent extraction. According to Canadian Patent 1 250 210, a solution containing iron and zinc is treated in two stages with metallic iron and zinc to reduce ferric to ferrous iron and cementation of copper, arsenic, antimony and bismuth on the iron, followed by precipitation of a sludge of tin, cadmium and lead in the second stage treatment with zinc dust. After this two-stage pre-treatment in an aqueous system, zinc chloride is extracted with an organic liquid. The reduction stages are, therefore, essentially separate from the solvent extraction process. It is noted that no metal is actually deposited onto the zinc powder in the second stage.
Shibata Junji et al. reported that ferric iron can be stripped from di-2-ethyl-hexylphosphoric acid (D2EHPA) with mineral acid and iron powder (Proc. Symp. Solvent Extr. 1986, 139-142). Shibata et al. only disclose the stripping of ferric iron from D2EHPA with iron powder. Shibata et al. do not disclose the galvanic stripping with metals other than iron, or the deposition of metals onto added metals, or stripping of metals other than iron from organics other than D2EHPA.
Taking the above-mentioned teachings according to the Canadian Patent and Shibata et al., one could not presume a priori that the Shibata et al. method is operable with zinc powder or metals other than iron and zinc. Similarly, it could not be presumed that the Shibata et al. method is operable with organics other than D2EHPA, or that actual deposition of a metal species dissolved in an organic liquid would occur in the organic phase onto an added solid metal. It could also not be presumed that addition of a solid metal to an organic phase would make it possible to reduce metal ions other than ferric ions in the organic phase from a higher to a lower state of oxidation.